The performance required for pneumatic tires includes lower rolling resistance and higher wear resistance as well as superior steering stability. To reduce rolling resistance of a formed tire, heat build-up of a rubber composition constituting a pneumatic tire has been suppressed. As an indicator of the heat build-up in a rubber composition, tan δ at 60° C. determined by dynamic visco-elasticity measurement is typically used, and smaller tan δ (60° C.) of the rubber composition indicates smaller heat build-up.
Examples of methods of reducing the tan δ (60° C.) of a rubber composition include reducing the compounded amount of carbon black, increasing the particle size of carbon black, and blending silica in place of carbon black. However, such methods are problematic in that the mechanical properties such as tensile strength at break, tensile elongation at break, and rubber hardness are diminished and that the steering stability, wear resistance, and durability of a formed tire are diminished.
Furthermore, recently, enhancement in performance of carbon black as well as silica has been desired to improve the wear resistance while the rolling resistance is further reduced.
For example, Japanese Unexamined Patent Application Publication No. 2004-519552 proposes reducing heat build-up in a rubber composition mainly by blending carbon black having an adjusted specific surface area (BET (Brunauer-Emmett-Teller) specific surface area, CTAB (cetyltrimethylammonium bromide) specific surface area, and iodine adsorption index IA), DBP (dibutyl phthalate) structure value, Stokes diameter dst, and the like. However, this rubber composition does not necessarily have an effect sufficient to ensure mechanical strength and wear resistance, and thus there has been a demand for further improvements.
On the other hand, construction vehicles, such as large dump trucks that operate at quarries and/or large scale construction sites, operate for a long time while carrying a heavy load. Large heavy duty tires mounted on such construction vehicles are required not only to have excellent cut resistance but also to prevent tire failure by suppressing overheating of the tire which is achieved by suppressing heat build-up (heat build-up resistance). Cut resistance is a property that makes it difficult to damage a tire when the tire is brought into contact or collision with a barrier or external object. Heat build-up is a property that mitigates the impact by converting the energy applied to a tire due to such a physical impact into heat for heating up a rubber. Because of this, in order to make the cut resistance of the tire excellent, large heat build-up of the rubber is required. On the other hand, in order to avoid overheating of the tire and failure due to the overheating, heat build-up of the rubber is required to be low. Thus, the cut resistance and low heat build-up are in a trade-off relationship.
The method of reducing the heat build-up of a rubber composition may include, for example, reducing the compounded amount of carbon black and increasing the particle size of carbon black. Such methods are problematic, however, in that the mechanical characteristics such as tensile strength at break and the rubber hardness are diminished and that the cut resistance and/or wear resistance of a formed tire is diminished.
International Patent Application Publication No. WO 2010/077232 proposes to blend silica, carbon black, a silane coupling agent, sulfur, and a sulfenamide accelerator at specific proportions with natural rubber in order to reduce the heat build-up of tires for large vehicles. However, this rubber composition does not necessarily have an effect sufficient to improve cut resistance. Therefore, further enhancement in both heat build-up resistance and cut resistance has been demanded.